This invention relates to an electrical circuit having multiple signal paths for processing electrical signals and, more particularly, to a circuit having selectable signal paths for processing signals received from different system hardware configurations or received for performing selected functions.
The invention is particularly useful in connection with instrumentation and control (I&C) systems of complex industrial process and power plants. Analog and digital components of I&C systems of complex industrial process and power plants tend to wear out after about five years or more and must be periodically replaced as long as the plants continue to operate. However, the development of integrated circuits having up to 50,000 (50K) gates or more on a chip (very large scale integration) and other dramatic improvements in the cost and performance of digital technology over the past ten to twenty years have so restructured the I&C field that many analog and digital components originally employed in vintage I&C systems are no longer available at acceptable prices or have been substantially redesigned. Accordingly, vintage I&C systems must be redesigned at ever increasing costs to utilize currently available components when the original components are discontinued.
Existing I&C systems may be completely replaced with modern full system upgrades to obtain the advantages of state of the art digital technology. The assignee of the present invention, for example, provides a digital I&C system (known as the Eagle 21 system) for commercial pressurized water nuclear reactors to upgrade existing analog systems, including the Foxboro H-line, Hagan and Westinghouse 7100 and 7300 and other I&C systems. Advantageously, the Eagle 21 system provides additional capabilities of automatic calibration, automatic system self-testing and additional computer data link channels. However, the substantial cost of a full system upgrade of a nuclear power plant even with its increased capabilities frequently is not justified when compared with the significantly lower cost of simply maintaining the current I&C system.
Where full I&C system upgrades can not be justified, programmable logic controllers or other microprocessor-based hardware may be employed to replace vintage analog and/or digital components and circuits. Thus, for example, B&W and Framatome provide their STAR microprocessor based modules and the Foxboro Company provides its SPEC 200 micro hardware to upgrade nuclear plant I&C systems. However, the Nuclear Regulatory Commission and the commercial nuclear power industry are very concerned about software common-mode failures of such devices and their sensitivity to plant environs. Thus, they require that the software be validated and verified before installation in nuclear power plants, which requires considerable time and effort.
Application specific integrated circuits (ASICs) were originally developed in the defense field for uses which require the high reliability and high performance of digital electronic technology without the perceived disadvantages of software. More recently, ASICs have been used in digital watches, communications and other mass markets where the ASIC related costs can be distributed over thousands of devices. General information regarding ASICs may be found in the "ASIC & EDA" magazine (which was retitled "Integrated System Design" as of the March 1995 issue). Other state of the art circuits and devices which do not require the use of software are available, including field programmable gate arrays (FPGAs), electrically programmable logic devices (EPLDs) and electrically programmable analog circuits (EPACs).
The electronics industry is presently able to produce an ASIC which can be economically employed in small markets such as I&C systems of nuclear power plants and chemical processing plants. However, these types of markets are frequently served by more than one vendor and each vendor will upgrade its products over time. Thus, ASIC based circuit boards and modules must be as functionally versatile as possible and must be able to interface with different system hardware configurations if they are to be useful in fragmented markets.